1. Field of the Invention
The present invention relates to a high pressure discharge lamp and to a method for producing the high pressure discharge lamp. More specifically, the present invention relates to a long-life high pressure discharge lamp which, even after being used for a long period of time, has a low degree of blackening and decrease in luminance, and which is capable of preventing leakage of a contained gas and blowout of the bulb, and to a method for manufacturing such a high pressure discharge lamp.
2. Description of Related Art
In general, a high pressure discharge lamp has a structure, for instance, as shown in FIG. 4. In the high pressure discharge lamp 110 shown in FIG. 4, each electrode of a pair of electrodes 102 and 102 made of tungsten is disposed so as to be opposite the other in a quartz glass bulb 101, which includes a round-shaped central portion. Each of these electrodes 102 and 102 is inserted from a respective insertion opening 104 located at an end of the bulb 101 and each of the insertion openings 104 is airtightly sealed with the respective electrode 102 via a sleeve-shaped molybdenum foil 105 which is a thermal cushioning material. A halogen gas, such as mercury gas or methylene bromide gas, and an inert gas, such as argon, are contained and sealed in the bulb 101.
In general, a relatively large amount of mercury, for instance, in an amount of more than 0.15 mg/mm3, is contained in the high pressure discharge lamp 110. When the lamp 110 is lit and a trigger voltage is applied to the electrodes 102 and 102, a glow discharge is induced between the electrodes under the atmosphere of the above-mentioned inert gas and the contained mercury is vaporized to emit light of high luminance and excellent color rendering property due to a plasma discharge by the high-pressure mercury vapor. Since light of high luminance and excellent color rendering property is obtained by using the high pressure discharge lamp as explained above, the lamp has recently attracted attention as a light source for devices such as a projection type liquid crystal display and is used for a variety of purposes.
During the initial phase of using the high pressure discharge lamp, problems associated with the use thereof were pointed out, such as blackening of the inner surface of the bulb and reduction in the luminance of the lamp after it has been lit for a considerably long period of time. These problems are attributed to the fact that, as shown in FIG. 4, tungsten atoms or molecules W are vaporized by the discharge which occurs at high temperatures and they are deposited onto the inner surfaces of the bulb 101. Accordingly, in order to prevent the generation of blackening, a halogen gas is used and is sealed in the bulb 101. The halogen gas produces halogen ions at high temperatures which bond to and vaporize the tungsten deposited onto the inner surface of the bulb 101 and redeposit the tungsten onto a base portion of the electrode at which the temperature is relatively low. This is a so-called “halogen cycle” and this cycle is repeated so that the generation of blackening of the bulb may be prevented.
A halogen compound, such as methylene bromide, is generally used as the above-mentioned halogen gas. The halogen compound, when the lamp is lit, is decomposed in the bulb 101 and generates halogen ions. In general, the halogen gas is contained so that the partial pressure of the halogen gas in the bulb 101 becomes 1×10−6 μmol/mm3 or greater which is considered to be an amount effective for preventing the generation of blackening.
Also, an inert gas, such as argon, is contained in the bulb 101 in an amount in the range between about 6×103 Pa and 6×104 Pa in order to induce a glow discharge at the start of lighting the lamp 110.
However, although a halogen gas is contained in the bulb 101 in order to prevent a decrease in the luminance of the lamp 110 due to the generation of blackening as mentioned above, the halogen gas, when present in excessive, tends to erode and deteriorate the electrodes 102 and molybdenum foils 105 at the sealing portions of the bulb 101. If the erosion proceeds, a contained gas may leak from the sealing portions or a blowout of the bulb 101 may be caused since the pressure inside the bulb 101 exceeds 100 atmospheres due to the vapor pressure of the contained mercury. For this reason, studies have been conducted to achieve an overall improvement of the high pressure discharge lamp, the improvement including the structure thereof and an amount of various components contained in the bulb 101 in order to prevent problems such as the generation of blackening, leaking of contained gases and blowout of the bulb 101.
For example, Japanese Unexamined Patent Application, First Publication No. 11-149899 discloses an amount of mercury contained between 0.12 and 0.35 mg/mm3, an amount of a halogen gas between 10−7 and 10−2 μmol/mm3, and an amount of potassium oxide contained in an electrode of 12 ppm or less (5 ppm or less in the embodiments). In this publication, it is concluded that the lower the amount of potassium oxide contained in the tungsten electrode, the greater the effect of preventing the generation of blackening of the bulb.
Japanese Patent No. 2829339 discloses a high pressure discharge lamp in which an amount of mercury contained is between 0.2 and 0.35 mg/mm3, and an amount of a halogen gas is between 10−6 and 10−4 μmol/mm3.
Japanese Patent No. 2980882 discloses an amount of mercury of 0.16 mg/mm3 or more, an amount of halogen gas between 2×10−4 and 7×10−3 μmol/mm3, and, preferably, a bulb wall loading of 0.8 W/mm2 or more and an amount of an inert gas of 5×103 Pa or more.
Japanese Unexamined Patent Application, First Publication No. 11-297274 discloses an amount of mercury which reaches between 100 and 200 atmospheres when a lamp is lit, and an amount of a halogen gas between 1.1×10−5 and 1.2×1−7 mol/cc.
Also, Japanese Unexamined Patent Application, First Publication No. 11-329350 discloses a discharge lamp filled with a noble gas, the ratio of the maximum intensity of the emission spectrum of hydrogen, oxygen and their compounds which are present in a light emitting part to the intensity of the main emission spectrum of the noble gas being {fraction (1/1,000)} or less, and the content of the hydroxyl group in the quartz glass of sealing parts being 5 ppm or less by weight.
However, no matter how the amount of components contained in the bulb of the high pressure discharge lamp is adjusted as described in the above-mentioned documents, problems of the decrease in the luminance of the lamp due to the generation of blackening, leakage of contained gas, and blowout of the bulb cannot be solved by any single means simultaneously.